This invention relates to a synchronization signal generating circuit which enables the adjustment of a period of a synchronization signal (hereinafter referred to as a sync signal) used, for example, in an image forming apparatus.
In a conventional image forming apparatus of the solid state scan system in which a photosensitive member is exposed to light emitted, for example, from light emitting diodes (LEDs) arranged in a line, there is provided a sync signal generating circuit as follows. A counter starts counting a reference clock output from a crystal oscillator using, for example, a copy start signal as a reference signal. Each time a count value reaches a preset value, the sync signal generating circuit generates a horizontal sync signal and sends the same to an image output unit. The image output unit outputs an image data in a main scanning direction to the LEDs in synchronism with the horizontal sync signal, so that the image can be formed.
However, an oscillation frequency of the crystal oscillator tends to vary slightly due to a production error or the like. If the variation of the oscillation frequency causes the period of the horizontal sync signal to shift from the one required in the image output unit, its shift amount increases gradually. As a result, the output of the image data from the image output unit cannot be synchronized with the horizontal sync signal properly, thereby causing the image distortion or the like.
Considering the above problem, it may be thought to change the set value for each crystal oscillator according to the variation of the oscillation frequency so as to obtain the horizontal sync signal of a desired period. However, the period of the horizontal sync signal can be set only as a multiple of the period of the reference clock. Thus, in the case where the period of the horizontal sync signal shifts from the period required as such by less than the reference clock, the horizontal sync signal of the required period cannot be obtained by changing the set value.